1. Field of the Invention
This invention relates to a power supply interruption detecting circuit for preventing occurrence of an error in a write signal or sending and receiving signals in a data processing unit in the case of AC power supply interruption.
2. Description of the Prior Art
When the computer was used only in the offline system at an early stage, it was sufficient merely to supply a CVCF power as the power supply of the computer system. But since the computer has been used in the online system, a non-stop CVCF power supply is required as a measure for power supply interruption because the online system does not allow of any interruption of its operation. However, the non-stop CVCF power supply perfectly fulfills its function as a non-stop power supply in the case of the commercial power interruption, but has no useful function in the case of a trouble of the power supply itself. Further, even if a reserve machine is provided for the power supply, the operation is interrupted in the case of switching from the main power supply to the standby facility. Moreover, if a power supply of good quality is produced by paying much attention to its design, working, materials and parts to provide for enhanced reliability in the power supply itself and if it is carefully maintained, troubles of the power supply can be decreased but it is impossible to exterminate the trouble. Furthermore, where a plurality of data processing units are connected to the same AC power supply, occurrence of a fatal accident in one of the units also interrupts the power supply to the other units for a short time.
By the way, the data processing units include a remote terminal unit, a teletypewriter, a printer, a punched tape card reader, a magnetic disc, a magnetic tape, etc., and almost all of them are those which send and receive signals between them or write signals on record media.
In such units, a direct current is produced from an AC power supply and a DC power is supplied to each electronic circuit in the units. In this case, in the transient state of connection and disconnection of the AC input, the normal DC output voltage cannot be retained and each electronic circuit in the unit cannot be maintained in its normal logic state. To avoid this, a special power on-off signal is used to clamp the sending and receiving signals or the write signal at a predetermined logic voltage, thereby prevent the signal from becoming an error signal.
With this method, however, where the AC power supply is cut off outside of the apparatus, it is impossible to prevent sending of an error signal from the data processing unit or write of the error signal.
Even if a power supply interruption detecting circuit is provided in one data processing unit to prevent false write and transmission, it is impossible to prevent reception of an error signal sent out from other data processing units.
In the prior art, a power supply interruption detecting relay is connected to the AC input circuit, and is adapted to be released upon cutting off of the AC input, thereby clamping the write signal or the sending and receiving signals with the relay contact. However, the level of the voltage detection by the release of the relay is not constant, and in anticipation of a long release time, the capacity of a smoothing circuit is required to be selected larger than is necessary so that a normal DC output voltage may be retained in the release time.